Hot press-forging of an optical body with dynamic constraint by crystal powder

Abstract

A method is disclosed for hot press-forging a normally frangible optically integral crystalline mass ("optical body") in a closed die, by directly pressing a surface of the mass while it is peripherally surrounded by a crystal powder the amount of which is computed from the deformation ratio desired and the geometry of the die. The crystal powder is compacted by the non-uniformly directed forces transmitted to it by the optical body, and eventually the crystal powder is sufficiently dense so as dynamically to constrain the optical body at its peripheral surface. The dynamic constraint of the optical body while it is being forged is effected in such a way as to permit deformation of the optical body without fracturing it, and, without affecting its optical integrity. Hot press-forging the optical body by peripherally surrounding it with the crystal powder produces a stress environment in which the forces applied in a first stage, to the optical body by the ram of the forging press and, in a second stage, by the crystal powder, are non-uniform both in magnitude as well as in direction. The result is that the optical body may be deformed and forged to obtain a deformation ratio greater than 1.0 so as to obtain a fine-grain structure free of peripheral fissures and "veiling". Typically, a melt-grown essentially single macrocrystal of an ionic salt or of an optical oxide of aluminum or magnesium including certain optical mixed oxides such as bismuth germanium oxide, cadmium tungstate and lithium niobate, is hot press-forged while it is surrounded by a crystal powder of the same ionic salt or oxide. A polycrystalline extrudate of a melt-grown macrocrystal may also be hot press-forged in an analogous manner.

I claim: 
     
       1. A polycrystalline press-forged composite, comprising (a) a central zone of an optically integral fully dense and homogeneous polycrystalline body being transparent in at least a portion of the wave length range from about 0.2 to about 30 microns, said body composed of a normally frangible material selected from the group consisting of an alkali metal halide, an alkaline earth metal halide, and an optical oxide, said body derived from an ingot or an extrudate and prepared by pressing the ingot or extrudate to obtain a deformation ratio greater than 1.0, and   (b) an outer zone surrounding said central zone and consisting of a cluster of compacted hot-pressed crystals having a density in excess of 98% of the fully dense central zone, said crystals having a particle size in the range of 20 microns to 250 microns and having essentially the same thermal coefficient of expansion as said material in the central zone.   
     
     
       2. The polycrystalline press-forged composite of claim 1, wherein the deformation ratio is from about 1.2 to about 10.